Rollup merge of #136761 - workingjubilee:specify-opt-level-for-codegen-tests, r=saethlin

tests: `-Copt-level=3` instead of `-O` in codegen tests

An effective blocker for redefining the meaning of `-O` is to stop reusing this somewhat ambiguous alias in our own codegen test suite. The choice between `-Copt-level=2` and `-Copt-level=3` is arbitrary for most of our tests. In most cases it makes no difference, so I set most of them to `-Copt-level=3`, as it will lead to slightly more "normalized" codegen.

try-job: test-various
try-job: arm-android
try-job: armhf-gnu
try-job: i686-gnu-1
try-job: i686-gnu-2
try-job: i686-mingw
try-job: i686-msvc-1
try-job: i686-msvc-2
try-job: aarch64-apple
try-job: aarch64-gnu

Author: GuillaumeGomez